What exactly does "in-hg" mean on a boost gauge?

I know it's vaccuum, but I'm wondering what the technical term is for it.

Thanks!

__________________"That the automobile has practically reached the limit of its development is suggested by the fact that during the past year no improvements of a radical nature have been introduced." (Scientific American January 2, 1909).

For those who want a bit more info, 2in/hg is close to 1psi, except psi is usually for pressure and in/hg for vacuum, hence the mixed units on gauges. 28in/hg is basicly absolute vacuum, which is why high rpm decel gets close to that.

You're the second guy who has told me that now, I'm starting to get confused damnit. Why in the hell would they say inches mercury.

__________________"That the automobile has practically reached the limit of its development is suggested by the fact that during the past year no improvements of a radical nature have been introduced." (Scientific American January 2, 1909).

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